In this page you will find our latest results on modified gravity. A full description of the work can be found here . In this page I describe the basic idea. There are mounting evidence that our Universe is expanding with a positive accelaration. To explain this expansion using the standard cosmology, which is based on general relativity (GR), we need an "unknown" form of matter called dark energy. Einstein called this form of matter as the cosmological constant. However, many theorists ask the question whether GR in its current form is the most general theory to describe gravity. So they modify GR in such a way that those modified theories can explain the accelarated expansion of the Universe without dark energy, ie. without a cosmological constant. On the other hand, GR is a well tested theory at different scales. One of the examples is perihelion of mercury. So any modified gravity theory has to be tested well before making statements against GR. Fortunately, these modified theories predict a varity of observational signatures which cannot be observed under GR. The most important is the extra 'fifth force'. Simply, this means that under modified gravity theories objects feel an extra force other than the simple Newtonian force. Therefore, by measuring the force between objects and compre that with the expected Newtonian force we can check the existence of modified force. Now, the question we ask can you feel the extra force any where in the Universe. The answer is NO. If the extra force exists in the Milky way (our galaxy) we would have already detected that. Therefore, one has to come up with an explanation. It was found that the extra fifth force gets screened in massive objects due some property of the extra field which causes the fifth force. So we need to figure out where we need to look for the extra force. It can be seen that, depending on the parameters of the theory, you can feel the extra force in isolated dwarf galxies. If the dwarf galaxy is under the influence of massive objects, like Clusters, groups or Milkyway like galaxies, then they do not feel the extra force as the massive objects screen the extra field. There are many ways you can detect the extra fifth force in isolated dwarf galaxies. For our study we use one elegant method, which is based of distance estimation to these galaxies using different standard candles. One of the major rulers for distance estimation in the nearby Universe is the Period-Luminosity of cepheid variables. In short, given the period of the cepheid these relation can predict its absolute luminosity. It can be shown that under modified gravity these cepheids oscilate faster than the normal gravity. Therefore, the infered distance based on the P-L relation under modified gravity is 3-12% smaller than the actual distance. So we can constrain the modified gravity by comparing the true distance to cepheid based distance. How do we measure true distance to the galaxy? It is found that one of the evolutionary stages of low mass star, known as Tip of Redgiant Branch, is good for such purpose for galaxies within 20 Mpc. For more pictures please see this link. I do not hold the copy right of these pictures |